In recent years, very small micromirror devices utilizing MEMS (Micro Electro Mechanical System) technology have been proposed, with a view to controlling the wavefront of light in the field of adaptive optics, and realizing a mechanical optical switch for reflecting a light beam in such a manner as to enter a photoelectric element or an optical fiber which is at a predetermined position.
In a micromirror device, it is necessary to enhance the planarity of the mirror surface in order to enhance the quality of reflected light. In order to realize this, use of a single-crystalline silicon layer of an SOI (Silicon on Insulator) substrate as a mirror surface has been proposed. A single-crystalline silicon layer which an SOI substrate possesses is characterized by absence of residual stress and excellent planarity, and allows a highly planar mirror to be formed. Moreover, vertical trenches can be formed in a single-crystalline silicon layer by DRIE (DEEP REACTIVE ION ETCHING) technique, and this can be utilized to form a mirror.
However, inside a single-crystalline silicon layer, it is difficult to form trenches which are parallel to the single-crystalline silicon layer, and it is impossible by merely processing the single-crystalline silicon layer to realize a hollow shape for allowing a driving electrode and a mirror to oppose each other. Therefore, in a micromirror device which requires the construction in which a driving electrode and a mirror oppose each other, a method is adopted which involves separately providing a substrate which has only a driving electrode formed thereon, and performing a bonding to integrate the mirror substrate and the electrode substrate (see, for example, Patent Document 1).
FIG. 24 is a cross-sectional view showing the construction of a conventional mirror array described in Patent Document 1 in a phase during its production steps.
At the phase shown in FIG. 24, a mirror substrate 100 and an electrode substrate 120, which have been separately produced, are interposed in a stacked state between a stage 901 and a die bonder 701. Specifically, the mirror substrate 100 is affixed upside down to the stage 901, whereas the electrode substrate 120 is affixed onto a die bonder 701.
The mirror substrate 100 is formed by patterning the SOI substrate 101 by photography technique and etching technique, and includes mirrors 105. The mirrors 105 are elastically supported by a frame portion (not shown), and the surrounding insulating layer and handle substrate are removed so as to permit free pivoting.
On the other hand, on the electrode substrate 120, control electrodes 124 are formed in positions opposing the mirrors 105. Supporting members 125 are formed on the electrode substrate 120, with a spacer pattern 801 being formed on the supporting members 125. The spacer pattern 801 is formed by, after separately forming an electrically conductive paste pattern (not shown) on a transfer plate (not shown), pressing it onto the supporting members 125 to effect a transfer. At this time, if the electrically conductive paste pattern is too thick, the spacer pattern 801 may be crushed and deformed, thus protruding. Therefore, the electrically conductive paste pattern is to be formed while its thickness is precisely controlled.
In order to form the multilayer structure shown in FIG. 24, the stage 901 carrying the mirror substrate 100 and the die bonder 701 carrying the electrode substrate 120 are allowed to make relative movements so that the spacer pattern 801 on the electrode substrate 120 abuts with a frame portion (not shown) of the mirror substrate 100. By performing a bake in this state, the mirror substrate 100 side and the electrode substrate 120 side are allowed to adhere to each other, whereby the mirrors 105 and the control electrodes 124 become fixed at a certain distance.
The control electrodes 124 are constructed so as to be capable of independent feeding via a wiring layer 123. For example, by setting the mirrors 105 at a ground potential, and applying drive potentials to the control electrodes 124, electrostatic forces can be created between the mirrors 105 and the control electrodes 124, thus causing the mirrors 105 to pivot.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-330363 (p. 9, FIG. 9)